1. Field of the Disclosure
This disclosure generally relates to oilfield exploration. More particularly, this disclosure relates to techniques for drawing fluids from a formation into a downhole tool.
2. Background of the Disclosure
“Heavy oil” or “extra heavy oil” are terms of art used to describe very viscous crude oil as compared to “light crude oil”. Such highly viscous crude oils are often referred to as “low mobility formation fluids”. Large quantities of heavy oil can be found in the Americas, in particular, Canada, Venezuela, and California. Historically, heavy oil was less desirable than light oil. The viscosity of the heavy oil makes production very difficult. Heavy oil also contains contaminants and/or many compounds which make refinement more complicated. Recently, advanced production techniques and the rising price of light crude oil have made production and refining of heavy oil economically feasible.
Heavy oil actually encompasses a wide variety of very viscous crude oils. Medium heavy oil generally has a density of 903 to 906 kg·m−3, an API (American Petroleum Institute) gravity of 25° to 18°, and a viscosity of 10 to 100 mPa·s. It is a mobile fluid at reservoir conditions and may be extracted using for example cold heavy oil production with sand (CHOPS). Extra heavy oil generally has a density of 933 to 1,021 kg·m−3, an API gravity of 20° to 7°, and a viscosity of 100 to 10,000 mPa·s. It is a fluid that can be mobilized at reservoir conditions and may be extracted using heat injection techniques, such as cyclic steam stimulation, steam floods, and steam assisted gravity drainage (SAGD) or solvent injection techniques such as vapor assisted extraction (VAPEX). Tar sands, bitumen, and oil shale generally have a density of 985 to 1,021 kg·m−3, an API gravity of 12° to 7°, and a viscosity in excess of 10,000 mPa·s. They are not mobile fluids where the formation temperature is approximately 10° C. (in Canada), and must be extracted by mining. Hydrocarbons with similar densities and API gravities, but with viscosities less than 10,000 mPa·s can be partially mobile where the formation temperature is approximately 50° C. (in Venezuela).
Various tools and techniques have been proposed to increase the mobility of a highly viscous formation fluid, such as heavy oils and bitumen, thereby to obtain a sample. The proposed techniques typically employ a single approach, such as coring into, applying heat to, or injecting a fluid into a formation in an attempt to retrieve a sample of the highly viscous formation fluid, regardless of the particular characteristics of the particular formation or viscous fluid. Tools which perform these techniques further typically execute a predetermined process, again without taking into account the characteristics of the particular formation makeup or fluid.